Iron Chemistry of a Pentadentate Ligand That Generates a Metastable Fe^III−OOH Intermediate

In an effort to gain more insight into the factors controlling the formation of low-spin non-heme Fe^III−peroxo intermediates in oxidation catalysis, such as activated bleomycin, we have synthesized a series of iron complexes based on the pentadentate ligand N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine). The following complexes have been prepared:  [(N4Py)Fe^II(CH₃CN)](ClO₄)₂ (1), [(N4Py)Fe^IICl](ClO₄) (2), [(N4Py)Fe^IIIOMe](ClO₄)₂ (3), and [(N4Py)₂Fe₂O](ClO₄)₄ (4). Complexes 1 and 2 have low- and high-spin Fe^II centers, respectively, whereas 3 is an Fe^III complex that undergoes a temperature-dependent spin transition. The iron centers in the oxo-bridged dimer 4 are antiferromagnetically coupled (J = −104 cm^-1). Comparison of the crystal structures of 1, 3, and 4 shows that the ligand is well suited to accommodate both Fe^II and Fe^III in either spin state. For the high-spin Fe^III complexes 3 and 4 the iron atoms are positioned somewhat outside of the cavity formed by the ligand, while in the case of the low-spin Fe^II complex 1 the iron atom is retained in the middle of the cavity with approximately equal bond lengths to all nitrogen atoms from the ligand. On the basis of UV/vis and EPR observations, it is shown that 1, 3, and 4 all react with H₂O₂ to generate the purple low-spin [(N4Py)Fe^IIIOOH]²⁺ intermediate (6). In the case of 1, titration experiments with H₂O₂ monitored by UV/vis and ¹H NMR reveal the formation of [(N4Py)Fe^IIIOH]²⁺ (5) and the oxo-bridged diiron(III) dimer (4) prior to the generation of the Fe^III−OOH species (6). Raman spectra of 6 show distinctive Raman features, particularly a ν(O−O) at 790 cm^-1 that is the lowest observed for any iron−peroxo species. This observation may rationalize the reactivity of low-spin Fe^III−OOH species such as “activated bleomycin”.